Invite (all) to the way of thy Lord with wisdom and beautiful preaching; and argue with them in ways that are best and most gracious: for thy Lord knoweth, best who have strayed from His Path, and who receive guidance. (Qur'an 16:125)
The concept of "Trinity" can be explained by giving the example that water can be present in three states, i.e. as solid e.g. ice, as liquid e.g. water and gas e.g. vapour, yet it is one and the same water.
I agree that water can be present in three states, that is solid, liquid and gas, as ice, water and vapour but in all three states the matter is the same i.e. H2O and contains two atoms of hydrogen and one atom of oxygen. The material and components are the same although the form of the matter differs. Water remains H2O in the solid form i.e. ice. It remains H2O in the form of gas i.e. vapour. However, in case of Trinity, besides the form changing even the components and material themselves change. The Father is God and Jesus (pbuh) is man, and the Holy Ghost is a spirit and all three are not made up of the same matter or component.
Man contains flesh and bones, whereas God and spirit do not contain flesh and bones. Man requires to eat, etc., whereas God does not require to eat. No wonder Jesus (pbuh) says in the last supper, in the upper room mentioned in the Gospel of Luke, chapter 24 verses 39 to 43
"Behold my hands and my feet, that it is I myself: handle me, and see; for a spirit hath not flesh and bones, as ye see me have."
And when he had thus spoken, he shewed them his hands and his feet.
And while they yet believed not for joy, and wondered, he said unto them, Have ye here any meat?
And they gave him a piece of broiled fish, and of an honeycomb."